The display system includes a system unit that receives a video signal output from an external signal source (host) and a display device that receives the video signal, output from the system unit, to display an image. The display device includes a display panel that displays the image, a drive circuit that drives the display panel, and a control circuit that controls the drive of the drive circuit. In recent years, a PSR technology is proposed as a technology of reducing power consumption of the whole display system (See Japanese unexamined published patent application No. 2013-190777).
In the PSR technology, in a case where image data (frame image data) in units of frames in the video signal output from the host is a still image, frame image data output operation of the system unit is stopped, and the display is performed using the frame image data stored in a storage unit of the control circuit. In the display system having the PSR function, the output operation of the system unit can be stopped while a still image is displayed, so that power consumption of the display system can be reduced as a whole.
However, in the display device applied to the display system, there is a problem in that display quality is degraded by a flicker. A principle of generation of the flicker will be described below. FIGS. 3, 19 and 20 are views illustrating a principle of the generation of the flicker.
In the PSR technology, in order to reduce the power consumption, a drive frequency at which a still image is displayed is set lower than a drive frequency at which a moving image is displayed. While the system unit is in a stopped state, the control circuit outputs the frame image data from the storage unit in asynchronization with the system unit. Therefore, timing at which the frame image data in the video signal output from the host is switched from a still image to a moving image deviates from timing at which a frame period of the still image output from the storage unit is ended. When the deviation is generated, a vertical retrace period (blanking period BR1) is lengthened in the frame image data (the image data of a frame 3 in FIG. 3) indicating a still image immediately before the switching from a PSR mode to a normal mode.
Generally, in a liquid crystal display, a phenomenon that a display luminescence is rising during a holding period after an image data is written in a pixel is occurred due to possible causes, such as properties and/or an orientation of liquid crystal and/or the like. FIG. 19 is a graph illustrating a result of measuring a display luminance on a display screen of a conventional liquid crystal display device with a photo sensor. FIG. 20 is a graph illustrating a change in display luminance on a display screen of a conventional liquid crystal display device.
When the blanking period becomes longer than or equal to a predetermined period (for example, a blanking period BR0 in FIG. 3), in a frame (a frame 3 in FIG. 3) immediately before the PSR mode is switched to the normal mode, a display luminance is higher than a predetermined luminescence because of lengthening the period of rising a display luminance (refer to FIG. 20). Then, in the frame (a frame E in FIG. 3) immediately after the switching from the PSR mode to the normal mode, a change in display luminance (display luminance difference) is increased, and recognized as the flicker by human's eyes.